One type of signal cable is a twisted pair cable. Each pair in such a cable consists of two insulated conductive wires twisted together. The wire pairs are twisted since it reduces crosstalk and noise susceptibility. An electrical conducting foil can be applied around each pair and work as a shield improving the crosstalk performance and stabilizing impedance.
Another type of signal cable is a twinaxial cable. A twinaxial cable consists of two insulated, non-twisted, conductors surrounded by an outer conductor. The outer conductor being usually a foil or similar and works as a conductive shield that reduces electrical noise from other signals of the cable as well as electromagnetic radiation. The entire assembly is then covered with an insulating and protective outer layer.
Twisted pairs and parallel/twinaxial pairs that are screened/shielded are frequently used in high-frequency copper links. The shield helps addressing crosstalk problems but puts very high requirements for balancing the symmetry of the cable. Even a small difference in capacitance of the signal wires leads to magnification of screen currents and rise of common mode propagation. The losses of energy of the differential signal to common mode not only reduces immunity of the link but also its propagation quality as the modes travel with different speed and have unpredictable frequency characteristics.
The conductive shield surrounding the insulated conductors of the above mentioned types of signal cables can be applied in various ways. One known solution for twisted pair cables is to helically wrap the conductive shield around the twisted pair in the same operation as the pair is twisted. This implies that the shield has the same lay length, i.e. the degree of twisting per unit length, as the pair itself. This result in that a longitudinal side of each wrap of the conductive shield overlaps the preceding wrap and that the overlap of the shield will be fixed in respect to the conductor's orientation. The overlap causes imbalances to be introduced, which degrades performance at high frequencies.
The conductive shield can also be helically applied to twinaxial cables. This introduces however structural impedance variations that create an upper limit for the usable frequency span. The periodic overlap causes a structure in which propagation of electromagnetic waves is deteriorated within a range of frequencies (stopband), whereby signals within this frequency range are attenuated. U.S. Pat. No. 7,649,142B2 discloses a twinaxial cable for high speed data communication with a helically wrapped conductive shield that overcomes some of these drawbacks. The conductive shield is applied using a tape with a variable width, which reduces the attenuation of signals having frequencies within a stopband by spreading the attenuation across multiple frequencies. Thereby the maximum attenuation of the signals in the stopband is decreased and spread out to frequencies outside of the stopband. The solution in U.S. Pat. No. 7,649,142B2 requires however potentially expensive, special types of conductive shield tape. Further, an increase in attenuation may appear in frequencies outside of the stopband.
In addition, cables with helically wrapped conductive shields experience a phenomenon known as “signal suck-out” or resonance, whereby high signal attenuation occurs at a particular frequency range.
A different way to apply the conductive shield is to apply the shield longitudinally. The shield is then not helically wrapped around the insulated conductors, but is applied longitudinally in a cigarette-wrap configuration with a longitudinal seam extending along the length of the cable. It is however difficult to manufacture cables using this method without imbalances to be introduced.
The known solutions of applying conductive shields to cables all result in one or more drawbacks irrespective of whether the conductive shields are applied in a helical or longitudinal fashion.